1. Antarctic fish Pagothenia borchgrevinki were held for seven days in sublethal concentrations of the water soluble fraction of diesel fuel oil. 2. The fish gills produced large amounts of mucus and there was an increased frequency of coughing. 3. Haematocrit increased due to an increased number of red cells. 4. Plasma osmolarity and chloride levels did not change. 5. The ability of the gills to extract oxygen from the medium at low PO2 was impaired.